1. Field of the Invention
The present invention relates to a method for crystallizing an amorphous silicon thin-film for use in a thin-film transistor (TFT) and a thermal annealing apparatus therefor, and more particularly, to a method for crystallizing an amorphous silicon thin-film for a TFT in which amorphous silicon thin-films on a large-area glass substrate for use in a TFT-LCD (TFT-Liquid Crystal Display) are crystallized uniformly and quickly by a scanning method using a linear lamp to prevent deforming of the glass substrate, and a method for fabricating a polycrystalline TFT using the same, and a thermal annealing apparatus therefor.
2. Description of the Related Art
To enhance driving speed and resolution and improve productivity through integration of driving circuits, replacement of amorphous silicon TFT by polycrystalline silicon TFT is being vividly performed under study. Difficulties confronting when fabricating a polycrystalline silicon thin-film are to prevent a deform of glass which is used as a substrate. To do so, amorphous silicon should be crystallized within a temperature and time at which the glass substrate is resistant without being deformed.
A metal-induced lateral crystallization (MILC) method proposed to overcome the above difficulties can lower an amorphous silicon crystallization temperature at 500.degree. C. or below and has advantages using simple equipment and processes compared with other crystallization methods. In this MILC method, a metal thin-film such as Ni, Pd and so on, is partially formed on the interface between the surface of an amorphous silicon thin-film and a substrate, and is thermally annealed at 500.degree. C. or so, in such a manner that crystallization proceeds at the portion where the metal thin-film has been formed and in lateral direction thereof. A polycrystalline silicon TFT can be fabricated using the above MILC, in which a device having an excellent electrical characteristic can be fabricated at 500.degree. C. or below.
FIG. 1 is a sectional view showing a manufacturing process of a TFT using the MILC method. As shown, an amorphous silicon thin-film 10 is formed in the form of an island on the whole surface of a glass substrate 100. Then, a gate insulation film 12 and a gate electrode 13 are formed in turn. Then, a metal film 14 of Ni is deposited on the whole surface of the substrate including a source region 10S and a drain region 10D and then annealed, to thereby crystallize a channel region 10C of the amorphous silicon thin-film 10 by the MILC method.
The above method has a shorter thermal processing time than that of a method for forming a gate electrode after depositing and crystallizing an amorphous silicon thin-film on the whole surface of a substrate. Since the above method crystallizes only a channel region, yield is considerably improved.
To crystallize an amorphous silicon by thermal annealing at the state where the metal thin-film is not formed requires thermal processing of about 30 hours at a temperature of 600.degree. C. or above. Meanwhile, the above MILC technique shows a crystallization velocity of 1.6 .mu.m/hr or more at 500 .quadrature..quadrature. or so, which must be very useful crystallization method. In the MILC method, when a thermal annealing temperature is 600.degree. C. or above, the lateral crystallization proceeds more quickly depending upon the temperature. Thus, the lateral portions of the portion where the metal thin-film has been formed are crystallized all by the MILC method.
Meanwhile, in the case of a next generation large-area glass substrate, it does not facilitate to implement a furnace thermal annealing apparatus and it is difficult to enhance productivity because of a long-term thermal annealing time. For this reason, a thermal annealing apparatus adopting a number of lamps shown in FIG. 2 has been proposed.
FIG. 2 is a sectional view schematically showing a lamp thermal annealing apparatus which is used for crystallization of an amorphous silicon thin-film according to the conventional prior art. As depicted, a bottom layer oxide film 22 is formed on a substrate 21. A Ni metal layer 24 is formed on the surface of an amorphous silicon thin-film 23 formed on the oxide film 22. Then, a process for thermally processing the amorphous silicon thin-film at high temperature for a second using lamps 29 and cooling it for five seconds is performed at least once, to thereby crystallize the amorphous silicon thin-film by the MILC method.
In the MILC method, only an opaque amorphous silicon thin-film is heated and crystallized and a transparent glass substrate is not heated by the lamps, to accordingly prevent a deformation of the glass substrate. The reason for cooling the amorphous silicon for five seconds or so is to block the heats of the heated amorphous silicon from being transferred to the glass substrate, in order to prevent deforming of the glass substrate due to the heats transferred from the amorphous silicon to the glass substrate.
However, the above method for heating the whole surface of the substrate is also limited to implement a thermal annealing apparatus for uniformly heating a large-area glass substrate, such as a substrate of 600 mm.times.500 mm or larger. As described above, if all the portions of the substrate are not uniformly heated, a thermal processing time should be longer in order to crystallize all the portions of the substrate. Thus, the temperature of the amorphous silicon may be locally raised, so that the substrate may be deformed.